1) Field of the Invention
This invention relates to a magneto-optical recording medium capable of writing, reading-out, erasing, etc. of information by irradiation with light such as laser beam, etc.
2) Prior Art
Magneto-optical recording has been recently regarded as an important means for information-rewritable optical disks, and rare earth-transition metal alloy films have been used for the optical disk recording media. However, the Kerr rotation angle of the rare earth-transition metal alloy films is as low as 0.3-0.40 as such and the methods for increasing the Kerr rotation angle by utilizing the light interference have been so far investigated.
FIGS. 4 and 5 are cross-sectional vertical views of magneto-optical recording media utilizing conventional methods for increasing the Kerr rotation angle.
In FIG. 5, light 6 incoming from the side of a transparent substrate 1 is reflected on the interface between a dielectric film 11 and a magnetic film 3 to cause a Kerr rotation. The light is reflected at the interface between the dielectric film 11 and the substrate 1, and again reflected on the magnetic film 11 to cause a Kerr rotation. By repetitions of multi-reflection in the dielectric film 11, the Kerr rotation angle is increased. However, the magnetic film absorbs a portion of the light without reflection, and thus the absorbance is also increased together with the increase in the Kerr rotation angle, resulting in a decrease in the reflectivity.
In case of the structure shown in FIG. 4, Faraday rotation effect of light passing through a magnetic film 3 and rotation of polarization plane by the multi-reflection in a second dielectric film 4 are added to the multi-reflection in a first dielectric film 2 as above, whereby the Kerr rotation angle is further increased, but the reflectivity is decreased.
In these two structures, an increase in the Kerr rotation and a decrease in the reflectivity take place at the same time, but a figure of merit F, i.e. a quantity proportional to a S/N (signal/noise) ratio of a medium, can be increased. When R is a reflectivity and .theta..sub.k is a Kerr rotation angle, the figure of merit F can be given by: EQU R.sup..alpha. .cndot..theta..sub.k, where .alpha.=1/2 or 1.
Prior art examples relating to the aforementioned methods are shown in Japanese Patent Applications Kokai (Laid-open) Nos. 61-17236, 59-152552, 57-16996, 60-63747, etc.
Japanese Patent Applications Kokai (Laid-open) Nos. 57-169996 and 60-63747 propose to make the refractive index of the first dielectric film 2 higher than that of the second dielectric film 4, as shown in FIG. 4. That is, it is proposed to increase the Kerr rotation angle by the multi-interference in the first dielectric film 2 and lower the refractive index of the second dielectric film 4, thereby making the apparent refractivity as a synthetic of those of a reflecting film 5 and the second dielectric film 4 approach to zero, thereby to increase the reflectivity.
In case of utilizing the interference in the first dielectric film 2, the Kerr rotation angle can be indeed increased, but the reflectivity is considerably lowered at the same time, and the figure of merit for read-out, F=R.sup..alpha. .cndot..theta..sub.k, where .alpha.=1 when the intensity of reflection is small, cannot be increased too much. Furthermore, the thickness of the first dielectric film 2 is fixed by conditions for maximum Kerr rotation angle and the thickness cannot be made larger, and the protection from corrosions due to oxidation, etc. of the magnetic film 3 cannot be improved. Still furthermore, the thickness of the first dielectric film 2 satisfying the conditions for maximum Kerr rotation angle becomes inevitably smaller, because of the larger refractive index of the first dielectric film 2.
In order to further increase the figure of merit in the structures shown in FIGS. 4 and 5, the reflectivity must be decreased, as explained above. Furthermore, the reflectivity is also utilized in the tracking or auto-focusing, and thus some intensity of reflection is required.
It is desirable from the viewpoint of corrosion resistance that the dielectric film 1 has a larger thickness, but the film thickness is restricted by the figure of merit in the prior art examples.